Prior to the present invention it has been known to convert 2-naphthol to 2,3-hydroxynaphthoic acid (2,3-HNA) by reacting the 2-naphthol with sodium hydroxide and then carboxylating the resulting sodium naphthoxide with carbon dioxide. It has also been known that the carboxylation tends to shift to the 6-position if potassium is used instead of sodium. Temperatures also appear to affect the formation of 2,6-HNA as opposed to 2,3-HNA.
After carboxylation, the proton at the carboxylation site is lost and is picked up by a second 2-naphthoxide molecule. Thus, for every mole of product formed, a mole of starting material is formed. This means the best possible conversion is 50%. Addition of potassium carbonate during the second step improves conversion. The products 2,6-HNA, 2,3-HNA and 2-naphthol can be separated by modifying the pH of an aqueous solution containing the three. Typically, 2,6-HNA can be isolated in this manner with 97-99% purity, with 2,3-HNA being the major impurity. It is important to adjust pH precisely and accurately as this can affect the yield and purity of the desired product.
The patent literature contains two processes for the carboxylation of potassium 2-naphthoxide. In one process, a flow of carbon dioxide is passed continuously through the apparatus during carboxylation, claiming a higher yield than in the process without continuous CO.sub.2 flow.